Generator power peak limiting in virtual synchronous machine wind turbine

Abstract

Limiting generator loads in a wind turbine having a power converter with a line side inverter configured as a grid forming inverter. A maximum grid current value is determined based on a measured generator load and a limit function. A virtual impedance value and a virtual voltage drop over the virtual impedance value are determined. A virtual synchronous machine angle is determined based on a power reference and the virtual grid power. The voltage reference for controlling the line side inverter is determined based on the virtual synchronous machine angle, the virtual voltage drop and the voltage magnitude reference.

The invention claimed is: 
     
       1. A method for limiting generator loads in a wind turbine comprising a power converter with a line side inverter arranged to supply power to a power grid, the method comprising:
 determining a maximum grid current value based on a measured generator load and a limit function which sets an upper maximum grid current value and a lower maximum grid current value such that (i) the maximum grid current value moves towards the lower maximum current value if generator load is greater than a maximum generator load and (ii) the maximum grid current value moves towards the upper maximum current value if the generator load is smaller than the maximum generator load or is smaller than a low maximum generator load; 
 determining a virtual impedance value based on a current difference between a current magnitude associated with a grid current and the maximum grid current value, where the virtual impedance value comprises a virtual resistance value; 
 determining a virtual voltage drop over the virtual impedance value based on the virtual impedance value and the grid current; 
 determining a virtual grid power based on (i) the virtual resistance value and the grid current or (ii) the grid current and a voltage magnitude reference for a desired grid voltage or a desired reactive power to be generated by the line side inverter; 
 determining a virtual synchronous machine angle based on a power reference for a desired power output of the power generating unit and the virtual grid power; and 
 determining a voltage reference for controlling the line side inverter to generate power based on the virtual synchronous machine angle, the virtual voltage drop, and the voltage magnitude reference. 
 
     
     
       2. The method according to  claim 1 , wherein determining the maximum grid current value comprises determining a load difference between the measured generator load and the maximum generator load, integrating the load difference, and determining the maximum grid current value based on the integrated load difference subject to the upper maximum grid current value and the lower maximum grid current value of the limit function. 
     
     
       3. The method according to  claim 2 , wherein integrating the load difference comprises multiplying the load difference with a gain factor which depends on the load difference. 
     
     
       4. The method according to  claim 1 , wherein determining the maximum grid current value comprises selecting the lower maximum grid current value if the measured generator load is greater than the maximum generator load, selecting the upper maximum grid current value if the measured generator load is smaller than the low maximum generator load, and determining the maximum grid current value by limiting a rate of change of changes between the upper maximum grid current value and the lower maximum grid current value. 
     
     
       5. The method according to  claim 1 , wherein the virtual synchronous machine angle is determined so that a double derivative of the angle is indicative of a deviation between a power reference and the virtual grid power combined with a damping power. 
     
     
       6. The method according to  claim 1 , wherein the virtual resistance value is determined based on a control algorithm, where the control algorithm comprises a proportional part which determines a value which is proportional with the current difference or an integral part which determines a value which is proportional with a time integral of the current difference. 
     
     
       7. The method according to  claim 1 , wherein a virtual reactance value of the virtual impedance value is determined dependent on a predetermined factor defining a ratio between the virtual reactance value and the virtual resistance value. 
     
     
       8. The method according to  claim 1 , wherein the current magnitude is determined as the maximum current magnitude of the current magnitudes of current phases of the grid current, or as the magnitude of a current vector of the grid current or a transformation thereof. 
     
     
       9. The method according to  claim 1 , where a virtual synchronous machine rotational speed is determined based on a combination of a feedback of a damping power, the power reference, the virtual grid power, and an inertial integration model, where the synchronous machine angle is determined based on an integration of the synchronous machine rotational speed, and where the damping power is determined based on the virtual synchronous machine rotational speed. 
     
     
       10. The method according to  claim 1 , comprising:
 obtaining a network voltage at a connection point of the output of the power generating unit; 
 determining a rotational speed of the grid based on the network voltage; and 
 determining a damping power based on a virtual synchronous machine rotational speed and the rotational speed of the grid. 
 
     
     
       11. The method according to  claim 1 , comprising:
 determining a modified voltage reference based on the voltage magnitude reference and the virtual voltage drop; and 
 determining the voltage reference based on the modified voltage reference. 
 
     
     
       12. The method according to  claim 1 , wherein the voltage magnitude reference is defined in a rotating direct quadrature frame. 
     
     
       13. The method according to  claim 12 , wherein the method comprises transforming the voltage magnitude reference to a transformed voltage magnitude reference in non-rotating target frame and wherein a modified voltage reference is determined by combining said transformed voltage magnitude reference with the virtual voltage drop. 
     
     
       14. A wind turbine control system arranged for limiting generator loads in a wind turbine comprising a power converter with a line side inverter arranged to supply power to a power grid, the wind turbine control system is arranged to perform an operation for limiting generator loads in the wind turbine, the operation comprising:
 determining a maximum grid current value based on a measured generator load and a limit function which sets an upper maximum grid current value and a lower maximum grid current value such that (i) the maximum grid current value moves towards the lower maximum current value if generator load is greater than a maximum generator load and (ii) the maximum grid current value moves towards the upper maximum current value if the generator load is smaller than the maximum generator load or is smaller than a low maximum generator load; 
 determining a virtual impedance value based on a current difference between a current magnitude associated with a grid current and the maximum grid current value, where the virtual impedance value comprises a virtual resistance value; 
 determining a virtual voltage drop over the virtual impedance value based on the virtual impedance value and the grid current; 
 determining a virtual grid power based on (i) the virtual resistance value and the grid current or (ii) the grid current and a voltage magnitude reference for a desired grid voltage or a desired reactive power to be generated by the line side inverter; 
 determining a virtual synchronous machine angle based on a power reference for a desired power output of the power generating unit and the virtual grid power; and 
 determining a voltage reference for controlling the line side inverter to generate power based on the virtual synchronous machine angle, the virtual voltage drop, and the voltage magnitude reference. 
 
     
     
       15. A wind turbine, comprising:
 a tower; 
 a nacelle disposed on the tower; 
 a rotor extending from the nacelle and have a plurality of blades disposed at a distal end thereof; 
 a control system configured to perform an operation for limiting generator loads in the wind turbine, the operation comprising:
 determining a maximum grid current value based on a measured generator load and a limit function which sets an upper maximum grid current value and a lower maximum grid current value such that (i) the maximum grid current value moves towards the lower maximum current value if generator load is greater than a maximum generator load and (ii) the maximum grid current value moves towards the upper maximum current value if the generator load is smaller than the maximum generator load or is smaller than a low maximum generator load; 
 determining a virtual impedance value based on a current difference between a current magnitude associated with a grid current and the maximum grid current value, where the virtual impedance value comprises a virtual resistance value; 
 determining a virtual voltage drop over the virtual impedance value based on the virtual impedance value and the grid current; 
 determining a virtual grid power based on (i) the virtual resistance value and the grid current or (ii) the grid current and a voltage magnitude reference for a desired grid voltage or a desired reactive power to be generated by a line side inverter; 
 determining a virtual synchronous machine angle based on a power reference for a desired power output of the power generating unit and the virtual grid power; and 
 determining a voltage reference for controlling the line side inverter to generate power based on the virtual synchronous machine angle, the virtual voltage drop, and the voltage magnitude reference.